1. Field of the Invention
This invention relates generally to air conditioning compressor systems and more particularly, to multistage centrifugal compressors designed to operate with a gaseous refrigerant entering at a nominal zero superheat level.
2. Description of Related Art
Presently, there is a need for small sized centrifugal compressors which are efficient and capable of being integrated in all types of systems, most typically, automotive systems. To make such small centrifugal compressors practical, it is necessary to use refrigerants which have lower vapor pressures and higher specific volumes than are encountered in conventional refrigerant systems, which typically employ piston, vane or scroll compressors. Furthermore, recent international legislation engendered by environmental concerns over the issues of global warming and ozone depletion have mandated the elimination of freons including those used in the multi-billion dollar air conditioning/refrigeration industry. Substitute refrigerants that have more beneficial environmental indices, such as R134 (a replacement for R12 which is widely used in the automotive industry) have been proposed for use in conventional air conditioning/refrigeration systems. Recently developed refrigerants, such as R134, have much higher specific volumes than conventional R12 and R22 fluids. Use of such recently developed refrigerants, however, requires a higher operating pressure ratio across the compressor which cannot be readily achieved with a single centrifugal compressor stage. Typically, conventional compressor systems utilize two (2) centrifugal stages and an electric motor intermediate the two (2) stages. Such systems are disclosed in U.S. Pat. Nos.: 2,793,506 , 3,859,815 and 4,105,372. The refrigerant enters the first, or low pressure, compression stage where it is partially compressed. The partially compressed gaseous refrigerant then passes through a diffuser and is collected in a scroll. The gaseous refrigerant then is transferred via an external tube to the inlet of the second, or high pressure, compression stage, where the compression is completed. There are significant deficiencies in these systems. It has been found that significant aerodynamic losses are incurred in the collection of the gaseous refrigerant in the scroll and the transfer of the gaseous refrigerant through the outlet of the low compression stage and into the inlet of the high compression stage. These aerodynamic losses are manifested through degradation of the coefficient of performance (COP) of the refrigerant cycle.
Additionally, in conventional systems, the motor assembly is typically cooled by extracting a small amount of liquid refrigerant from the condenser and flashing it in passages in the motor assembly. The vaporization of the refrigerant supplies the requisite cooling. However, it has been found that degradation of the COP of the refrigerant cycle occurs when the gaseous refrigerant is returned to the main flow of the gaseous refrigerant at an intermediate station in the compressor. Alternatively, the gaseous refrigerant can be injected back into the suction line which couples the evaporator outlet and the compressor input. Superficially, this would appear to augment necessary superheat in the cycle and thus, not cause degradation in the refrigerant cycle. However, it has been found that since the compressor inlet in this type of cycle is sub-atmospheric, the suction line between the evaporator and the compression inlet must be very short to avoid excessive inlet pressure losses. Consequently, the evaporator and the compressor must be closely coupled, thereby making it very difficult to inject this waste refrigerant into the system at this point without incurring additional losses due to the compressor inlet distortion.
Conventional centrifugal compressors typically utilize D.C. (direct current) or low frequency A.C. (alternating current) electric motors. However, it has been found that vehicle performance is adversely affected by the heavy weight and voluminous size of these motors.
It has also been found that the utilization of transfer tubes to execute the transition of the refrigerant from the low pressure compression stage to the high pressure compressor stage imposes significant restrictions with respect to the design geometry of the compressor system and its integration with other systems, such as automobile engines. Such restrictions are contrary to automobile industry design criteria which specifies that primary air conditioning components be on or substantially adjacent the vehicle center line. Hence, compressor systems having a left side/right side drive capability would be preferred over systems having design geometry of the conventional systems. It is possible to design a substantially cylindrical configuration for the systems disclosed in the above mentioned U.S. Patents, however, the diameter of the compressor would significantly increase. Additionally, it has been found that the relatively large wetted surface area in such a configuration would contribute to unacceptable high pressure losses and thus, cause a deleterious impact on the refrigerant cycle.
Finally, it has been found that conventional compressors may allow gaseous refrigerant containing liquid to enter the compression stages. In conventional systems, the only remedy for this is to operate the evaporator at a level significantly above zero superheat level. However, this degrades the overall system performance because it is an energy-inefficient remedy.
Bearing in mind the problems of the prior art, it is therefore an object of the present invention to provide a new and improved centrifugal compressor having two (2) sequentially arranged centrifugal compression stages.
It is another object of the present invention to provide a new and improved centrifugal compressor that is smaller in size than conventional compressors.
It is a further object of the present invention to provide a new and improved centrifugal compressor wherein the electrical motor assembly is cooled by gaseous refrigerant directly emitted by an evaporator.
It is another object of the present invention to provide a new and improved centrifugal compressor wherein the processes of convection and conduction are utilized to heat the incoming gaseous refrigerant so as to remove any liquid therefrom, thereby permitting the evaporator to operate at a zero superheat level.
It is another object of the present invention to provide a new and improved centrifugal compressor wherein the geometric shape of the compressor allows it to be integrated into an automobile engine system at a point on or substantially adjacent the vehicle center line.